A 1987 graduate of Cornell University, Knowles was a defensive end on the Big Red football team.

On December 28, 2009, Knowles was named the defensive coordinator at Duke under head coach David Cutcliffe.

[2] On January 29, 2018, Knowles was hired as the defensive coordinator at Oklahoma State under head coach Mike Gundy,[3][4] and held the position through the 2021 season.

[20] Ohio State Buckeyes head coach Ryan Day announced on December 7, 2021 that Knowles had accepted the defensive coordinator position for his team, effective January 2, 2022.

[22] Knowles helped Ohio State win the National Championship that same season, the first in the new 12-team College Football Playoff format.